The present invention relates to a Petri dish for cultivating micro-organisms, comprising a receptacle with a revolution shape receiving a nutritive substrate and on which rests a lid. A particularity of this Petri dish, in a specific embodiment, lies in the presence of several protrusions or fingers at the periphery of the lid, and of “attachment points” at the periphery of the receptacle receiving the protrusions of the lid.
The person skilled in the art in the field of techniques for microbiological analyses uses Petri dishes of different sizes for cultivating micro-organisms (diameters generally varying from 50 to 150 mm). The most common materials used for making these dishes are transparent plastic materials such as crystalline polystyrene (PS).
There exist two general types of Petri dishes, i.e. dishes with lugs, the receptacle of which has protrusions on which rests the lid, thereby allowing gas exchange with the inner space of the dish, and dishes without lugs not allowing gas exchange with the inner space of the dish. The person skilled in the art is aware of the benefit of managing gas exchanges for microbiological cultivation. As examples of functions fulfilled by these lugs, mention will be made of:                the gas exchange required for drying the dishes upon hot filling the receptacle of the Petri dish with nutritive substrate (gelose),        the ventilation allowing replacement of oxygen present in the atmosphere of the internal space of the dish for cultivating anaerobic germs.        
In the case of dishes without lugs, the lid rests directly on the lower receptacle, thereby limiting gas exchanges. In this case, the dryness of the nutritive substrate is lower and thus allows the nutritive substrate to be preserved for a longer time. The absence of lugs also allows an increase in the incubation time of the dish without any dryness, thereby guaranteeing better growth of “stressed” micro-organisms or with slow growth. Certain types of Petri dishes moreover allow the dish to be closed after sampling through a locking system (a lid attached by mechanical tightening or mechanical locking by rotation or clipping) so that the dish will not open by itself in order to limit risks of subsequent contaminations after sampling.
Within the scope of the present invention, the term of “sampling” has the following definition:                in the case of surface inspections, the sampling consists of applying gelose (nutritive substrate) contained in the dish on the surface to be inspected;        in the case of air inspections, the sampling consists of opening the dish in order to collect the air to be analyzed on the gelose (either by impaction or by passive sedimentation of air on the dish);        in the case of a microbiological inspection of a product or a micro-organism and culture, the matter will be to sow the gelose with strain or the product to be cultivated.        
However, if the solutions proposed up to now allow locking of said dish in the case of a fall or tampering, it is impossible to guarantee that the dish received for cultivation has not been opened and it is therefore not possible to guarantee the security of the result and the absence of “recontamination”, in fact inducing false positive results. An example of such a Petri dish is illustrated in document EP-0 171 174.
The present invention aims at solving these difficulties. In other words, its goal is to provide a Petri dish with which it is possible to ensure some traceability of the different handling operations which have been performed on the lid relatively to the receptacle (or vice versa), and most particularly when the latter have been locked beforehand. Thus, as an example, there exists an expectation for a Petri dish which, while the lid and receptacle occupy a position preventing gas exchanges, immediately informs the operator of a misguided manipulation during which this lid and this receptacle would have been separated and then brought back into the initial position.
Thus, according to the invention, we are therefore dealing with a Petri dish, which consists of a receptacle and of an additional lid, which both have a revolution shape and which are each delimited by a bottom wall and at least one peripheral wall,                a wall of the receptacle or of the lid respectively bearing at least two angular equidistant fingers, generally parallel to the bottom wall, which radially protrude towards the wall of the lid or of the receptacle respectively,        while the wall without any fingers bears a same number of locking members provided with a cam path able to receive said fingers,        this receptacle and this lid may selectively occupy either one of the different successive positions indicated below:                    a/ a “first position”, a so-called “non-locked beforehand” position, in which said fingers are not received into said locking members;            b/ at least one “intermediate position” selected from:                            a “non-ventilated locked” position, in which said fingers are received in said locking members and in which the receptacle is in intimate contact with the lid, so that the internal space which they delimit is isolated from the outside medium, and                a “locked and ventilated” position in which said fingers are received in said locking members and in which the receptacle is not in intimate contact with the lid, so that the internal space which they delimit is not isolated from the outside medium;                                    c/ a “final position”, a so-called “read out” position, in which said fingers are released from said locking members and allow the lifting of said lid,                        
the passing from one position to the next position being accomplished at least by relative rotation of the lid relatively to the receptacle, in a same direction.
This dish is remarkable in that said locking members bear at least one means for blocking the fingers in the “intermediate position”, which opposes return to the “first position” while allowing passing to the next position. By the presence of this blocking means, it is normally not possible to bring the receptacle and lid back into the preceding position. This “locking” ensures the operator of the “status” of the dish being in the “intermediate position”.
Moreover, according to other advantageous and non-limiting characteristics:                said blocking means is an anti-return ratchet;        said fingers have a region forming an interior angle, capable of forming an abutment for the ratchet;        said anti-return ratchet is retractable;        said cam path is delimited by the wall itself and by a “lug” of material secured to the wall, this wall and this “lug” delimiting transversely, i.e. along a direction generally perpendicular to the direction of the relative displacement of a finger, a corridor for entering said finger and guiding it along the cam path between the “first position” and the “final position”;        it includes indicating means positioned at the entry of said corridor, and/or at its exit, the initial condition of which is modified upon passing from the “first position” to the “intermediate position”, from the “intermediate position” to the “final position”, respectively, this change of state being visually perceptible;        said indicating means comprise at least one tab secured to the wall and to the lug which, in the modified state, is detached from one of said wall and lug;        in the modified state, said tab is secured to said lug and is immobilized in the raised position;        said lug is retractable outwards, i.e. in a direction generally opposite to the lid;        said other wall bears a protrusion which, upon passing from the “intermediate position” to the “final position”, exerts a force on said lug and retracts it.Other features and advantages of the invention will become apparent upon reading the following detailed description of preferred embodiments.        